Suspensions of alkaline-neutralizing agents may be used in a variety of diverse applications. Alkaline-neutralizing agents may be, for example, caustic solutions or lime slurries. Lime slurries can commonly be used in a water-softening process called “lime softening” where lime causes ‘hardness’ ions to precipitate from and settle out of solution, thus softening the water. In wastewater and process water applications, it can be desirable to use lime slurries instead of conventional caustic (often prepared from sodium hydroxide, or “NaOH”) solutions to adjust the pH of, or to neutralize acidic solutions on a commercial, municipal, and/or industrial scale.
In order to make these suspensions of alkaline-neutralizing agents, an alkaline chemical may be required. An alkaline chemical (those chemicals having a pH greater than 7) may be a chemical like NaOH or lime. To neutralize acidic solutions of both raw and treated water, caustic solutions can be used. These high-concentration caustic solutions (for example, about 50% NaOH with the balance as water) can be typically used for many of these applications due to the difficulty in handling lime and its traditional slurries, described in summary below. However, caustic solutions can be highly dangerous and must be carefully controlled to ensure that humans do not come into contact with these solutions, which can cause severe burns. In addition, the solid NaOH particles that often can be used to make caustic solutions by dissolution in water also present serious health concerns, and their use, shipment, and storage must be carefully controlled; they are subject to numerous safety regulations. Lime can be considerably less expensive than NaOH and also may be preferable from a health-safety standpoint, but handling problems associated with lime typically overshadow its use. These alkaline-neutralizing agents may contain lime particles, which can be suspended in lime slurries. One example of lime slurry usage can be in sewage and wastewater treatment, where alkaline-neutralizing agents may commonly be used to treat large amounts of water that have an acidic pH, or to increase the pH of the water to kill bacteria, microbes, and/or other organisms.
Lime slurries may sometimes be used as a substitute for caustic solutions to raise or neutralize the pH of commercial-scale acidic solutions such as treated wastewater. Unlike NaOH solution, these slurries may not be true solutions but instead can be suspensions of solid particles of hydrated lime in water. To make a lime slurry, particles of quicklime (chemically, calcium oxide or CaO) can be added to a water carrier, wherein the quicklime particles are hydrolyzed to produce particles of hydrated lime (Ca(OH)2). Alternatively, dry hydrated lime, Ca(OH)2 can be slurried with water to make a hydrate slurry.
Lime slurries may exhibit comparable reducing or alkaline-pH neutralizing power as alkali-metal hydroxide solution caustic agents while not requiring the addition of potentially hazardous alkali-metal hydroxides to the slurries. A conventional lime-based caustic-replacement slurry may include a quantity alkali-metal hydroxide in the slurry.
To approach the neutralization power of conventional caustic solutions, very-high solids lime slurries may be used, for example about or exceeding 30% hydrated lime solids by weight. But using such high-content lime solids can be problematic because the resulting viscosity can render the slurries impractical or un-useful from a materials-handling standpoint. One way to reduce the viscosity of a high solids lime slurry may be to add gypsum.
Another method to moderate the viscosity of a high-solids content hydrated lime suspension may be to incorporate a polymeric dispersing agent. For example, certain polyacrylic acids have been used as dispersing agents to moderate the viscosity of such a high-solids suspension. Other methods of making an alkaline-neutralization agent with high solids have been outlined in several patent applications. Some examples of these high solids lime slurries have been outlined in U.S. Pat. No. 7,718,085 and U.S. Pat. No. 7,897,062. However, creating lime slurries, especially a high solid version, can lead to scale. One issue with using lime slurries can be scaling with the equipment. For example, the increase in solids can result in very high viscosities and can also lead to increased scaling in the slurry-producing and conveying equipment as well as in other areas within the process. This scaling can occur within the atomizers of a spray dry absorber (SDA). The scaling can lead to maintenance issues, feed rate decrease, density changes, and reduction of stability in the sulfur oxide (SOX) levels in the SDA. Other inconsistencies not listed herein may also occur within the lime slurry process. In addition, scaling may also increase lime usage.
Therefore, it is desirable to develop the process in which scaling may be reduced in the production of a lime slurry in order to reduce these issues associated with scaling described herein.